The present invention relates to communication antennas and, in particular, to a bi-directional, dielectric loaded, conical horn antenna, for point-to point communications, particularly home and commercial satellite. Interiorly, the antenna body includes a plurality of conical stages of successively increasing flare angles, hybrid mode producing discontinuities and electromagnetic collimating apparatus.
Critical to the performance of any electromagnetic communication system are its transmitting and receiving antennas. The transmitting antenna is used to direct or focus radiated power in a desired direction toward a receiving antenna which is mounted to detect the transmitted radiation, while receiving a minimum amount of noise from sources radiating along adjacent axes. The use of directional antennas exhibiting relatively high on-axis gain and minimal off-axis side lobes or other undesired signal characteristics enhance the ability to communicate point-to-point. A further desired attribute of such antennas is an ability to focus or amplify the free-field radiation without cross-polarization, since most communication channels use two linearly polarized signals whose electric fields are oriented at right angles to one another.
With the above in mind and appreciating the high cost per unit area of paraboloidal reflector antennas and avowed interest in developing television broadcast and/or data communication systems using satellites in geostationary orbit--not to mention systems for satellite communications, radar and radio astronomy and terrestrial communications-- considerable interest exists to develop improved antenna systems of high directivity. Appreciating also that there is only one geostationary orbit, the Clarke orbit, only a finite number of satellites can be positioned in this orbit. It will therefore be necessary to space the satellites as closely as possible.
Improved ground station antennas will consequently be required. These antennas should radiate or receive circularly polarized planar wave fronts with high gain and directivity relative to the longitudinal axis of the antenna. Losses at the receiving aperture and over the length of the antenna should be minimal. Transmissions should further exhibit low side lobe levels to desirably avoid interference with transmissions between adjacent satellites and the earth.
The cross-polarization radiation level of transmissions should also be kept low. That is, antenna transmissions should have equal "E" and "H" plane radiation patterns. This will allow signals to be transmitted/received on opposite polarizations, which will enable diverse applications wherein communication standards require sending signals of different polarizations.
For satellite communications and other special applications, the transmitted/received energy beam should also be steerable. An antenna configuration with a variable beamwidth facility is preferred. The antenna configuration should accommodate a relatively wide band of frequencies, specific frequency ranges being accommodated with scaling or sizing adjustments to the antenna. Antennas for radio astronomy applications should exhibit the combined features of low cross polarization, suppressed side lobes, beamshaping and wide bandwidth, in addition to relatively high on-axis gain and improved directivity.
Reflector antennas, which are commonly used to receive microwave and shorter wavelengths, provide a relatively large reflective parabolic collector and exhibit broad-band gain characteristics. They also include a rear facing feedhorn capable of receiving broad beamwidths. The feedhorn is typically aligned with the signal axis and focal point of the collector to receive the focused signal and direct it to associated receiver electronics which appropriately convert and amplify the signal for its intended application.
Although the collector of these antennas is constructed to receive and focus the primary signal, undesired side lobe signals are commonly received due to necessarily broad collector and feedhorn acceptance angles. These side lobes are more prevalent as the receiving antenna is positioned further and further from the equatorial orbit, which correspondingly reduces the reception angle, causing greater amounts of ground noise to be collected with the focusing of the antenna.
Applicants have found however that over a number of bandwidths, centered on frequencies corresponding, for example to "C" and "KU" microwave bands, a forward-facing, multiple section conical antenna having a relatively narrow acceptance aperture, high gain and low side lobe characteristics can be used by itself, independent of a large surrounding collector. This entire antenna is of a physical size comparable to the feedhorn only of many current reflector antennas. The housing construction of this antenna is particularly described in Applicant's U.S. application Ser. No. 295,805 entitled Multimode Dielectric-Loaded Double Flare Antenna, filed Jan. 11, 1988. For the interested reader and as regards the geometries of the antenna, Applicants direct attention thereto.
To the extent Applicants are aware of antenna designs including features bearing some similarities of appearance to those of the subject invention, Applicants are aware of U.S. Pat. Nos. 2,761,141; 3,518,686; 3,917,773; and 3,866,234. These references generally disclose externally mounted dielectric antenna lenses of various shapes.
Applicants are also aware of U.S. Pat. Nos. 2,801,413; 3,055,004; 4,246,584; and 4,460,901 wherein the use of dielectric structures in association with horn antennas are shown.
Relative to multi-flared feedhorn antenna designs, Applicants are also aware of U.S. Pat. Nos. 2,591,486; 3,898,669; 4,141,015; and 4,442,437 which disclose various rear facing reflector antenna feedhorn designs. Also disclosed are stepped discontinuities within the antenna horn. The 3,898,669 patent additionally discloses a multiflare rectangular horn antenna. None of the noted references however are believed to disclose the presently claimed combination of features for producing an antenna adaptable to a variety of frequencies, most particularly KU and C microwave bands, and/or antennas utilizing dielectric insets or electromagnetic collimators of the configurations and compositions of the present invention.
Applicants are also aware of two papers authored by one of Applicants which are descriptive of reflector antenna feedhorn constructions. These are Nair, R. A., et.al; "A High Gain Multimode Dielectric Coated Rectangular Horn Antenna", The Radio and Electronic Engineer (IERE), London, September 1978, pp. 439-443 and Nair, R. A., "Radiation Behavior Of A Dielectric Loaded Double-Flare Multimode Conical Horn With A Homogeneous Dielectric Sphere In Front Of Its Aperture", Proceedings of the 1986 Montech Conference (IEEE), Quebec, Sep.29-Oct. 3, 1986. Neither paper however discloses the following described combinations or singular features of homogeneous or heterogeneous dielectric collimators--conical or otherwise--that mount interiorly of the antenna horn body. The present insets also exhibit minimal contact with the electrically conductive horn interior.